1. Technical Field
This disclosure relates generally to impellers for centrifugal slurry pumps. Slurries are usually a mixture of liquid and particulate solids, and are commonly used in the minerals processing, sand and gravel and/or dredging industry.
2. Background Art
Centrifugal slurry pumps generally include a pump housing having a pumping chamber therein which may be of a volute configuration with an impeller mounted for rotation within the pumping chamber. A drive shaft is operatively connected to the pump impeller for causing rotation thereof, the drive shaft entering the pump housing from one side. The pump further includes a pump inlet which is typically coaxial with respect to the drive shaft and located on the opposite side of the pump housing to the drive shaft. There is also a discharge outlet typically located at a periphery of the pump housing.
The impeller typically includes a hub to which the drive shaft is operatively connected and at least one shroud. Pumping vanes are provided on one side of the shroud with discharge passageways between adjacent pumping vanes. In one form of impeller two shrouds are provided with the pumping vanes being disposed therebetween. The pump impeller is adapted to be run at different speeds to generate the required pressure head.
Slurry pumps are often required to be of a relatively large size with large diameter and width impellers. These pumps need to have relatively large discharge passageways in order to facilitate the passage of larger solids within the slurry and reduce the overall velocity of the slurry as it passes through the impeller. Slurry pump parts are subject to significant wear from the particulate matter in the slurry. As a result of this the number of pumping vanes is small, e.g. three, four or five. To try to reduce wear, slurry pumps are typically operated at relatively low speeds, e.g. 200 rpm up to 5000 rpm for very small pumps. The materials used for slurry pump parts are generally very hard metals or elastomeric materials which are adapted to be sacrificed and subsequently replaced. In order to change the pump performance in terms of flow and pressure head, centrifugal pumps can achieve this by variation of the pump speed.
Centrifugal slurry pumps often need to be capable of use over a wide range of flow and pressure head conditions. The performance of centrifugal slurry pumps may be adversely affected by the size, density and concentration of the particulate matter within the slurry and the pump performance will also be affected by wear. The need to be able to operate a slurry pump over a wide range of conditions means that, because of the larger passageways in the impeller, the pump performance does tend to vary substantially and provide less guidance to flow through the impeller, compared with a smaller and narrower water pump which provides good flow guidance. Particles and liquid in the slurry also tend to take different paths through the impeller depending on the particular particle size and the concentration in the slurry. This phenomenon will be exacerbated by wear of the impeller. Centrifugal pumps often suffer from loss of flow because of slip at the periphery of the impeller and recirculation at the inlet and outlet of the impeller. Vortex style flow patterns can be established in the discharge in the impeller at lower flows. Such phenomena normally result in poorer pump performance.
A further phenomena associated with centrifugal pumps is that of cavitation, which occurs mainly in the pump intake and impeller intake and which can affect pump performance and may even cause damage to the pump if the cavitation is strong and continuous. As mentioned, centrifugal slurry pump parts are made from hard metals or elastomeric materials which are difficult to cast or mould and, as such, in order to simplify the manufacturing process, the impeller shrouds are generally arranged more or less parallel to one another at a constant distance apart from the inlet to the outlet. Because of this, the outlet of the slurry pump impeller is also subjected to recirculation, vortex flow and flow patterns which induce wear.
There are other types of fluid machines which utilise rotating elements for transferring fluid. Examples of such machines include centrifugal compressors, turbines, and high speed water pumps. The design considerations and criteria for apparatus of these types are quite specific to such machines, are better understood, and are relatively easy to apply. Gases have a low density and generally no entrained particles, and can be pumped at much higher velocities within the fluid machine. As friction is a minor component in a gas machine, turbulence can be minimised by using multiple vanes or splitter vanes. Vanes used in these types of fluid machine are all relatively thin because these vanes are not subject to erosive wear. Furthermore, and most importantly, splitter vanes function in effect in a similar fashion to the main vanes to increase or add energy to the gaseous flow. The splitter vanes are usually slightly shorter than the main vanes so as not to interfere with flow at the leading edge of the main vanes.
Secondary (or splitter) vanes are normally of the same configuration as, but somewhat shorter than, the main vanes and are positioned approximately midway between the main vanes. These splitter vanes function to split the flow into smaller passageways and add more guidance to the flow, thus minimising turbulence. This type of gas machine typically operates at very high speeds in the order of 50,000 to 100,000 rpm. The number of blades is normally quite high, say 20, and there could be splitter vanes in between, so the vanes therefore need to be thin and the passageways small. Splitter or secondary vanes are normally of the same height as the main pumping vanes to allow maximum guidance and maximum energy to be input (or taken out) of the fluid as it passes through the rotating element of the machine.
High performance water pumps are similar in some ways to centrifugal compressors or turbines, and some of the same strategies are applicable such as a high number of vanes (typically 7 or higher), and splitter style vanes between the main vanes to control turbulence and/or to smooth the outlet pressure pulse by having a high number of vanes. In use this results in a higher number of smaller pressure pulses from each vane. Water pumps are not used to pump particles and so do not require high wear resistant materials. Typical high performance water pumps also run at higher speeds than standard water pumps and can run at speeds of 10,000 to 30,000 rpm.
The greater the number of main pumping vanes, the lesser the pressure pulse from each vane. To reduce the overall pressure pulse from a fluid machine it is known that increasing the number of vanes will smooth the pulse. This is why some water pumps and gas compressors have a larger number of vanes, and why splitter vanes are added to double the number of vanes. The design criteria for machines a gas compressor, turbine or high performance or high-speed water pump have no relevance to that of slurry pumps.
The provision of extra guidance or attempting to reduce turbulence by adding a higher number of thinner vanes or reducing the passageway size through an impeller is counterproductive in the design of a slurry pump. The very things that improve the performance in the machines of this type will not offer any solution when applied in a slurry pump.
Centrifugal slurry pumps are quite unique fluid machines because it is necessary to balance design, wear and manufacturability in different wear resistant materials. As discussed earlier it is normally necessary to develop a slurry pump that operates over a wide flow and speed range so that it is applicable to a wide range of applications, but this makes it more difficult to optimise the design. Typical designs are robust, but being a fluid machine, such pumps still suffer loss of performance and wear due to internal turbulence. Due to the special and restricting design constraints, various strategies have been used to improve performance but these have met with rather limited success. Design strategies to minimise turbulence are quite difficult given the minimum guidance that the slurry can be given by the impeller shroud, main vanes and casing as all of these components need to have satisfactory wear life.
An additional complication with slurry pumps is that the particles in the slurry do not follow the fluid streamlines. The larger and more massive the particle, the greater the particles deviation from the fluid streamline. Consequently, adding more vanes (or splitter style vanes) that are designed to guide the fluid along streamlines is not going to assist to guide the particles because the particles will simply cause increased turbulence and wear on any thin vanes and these vanes will quickly become worn and lose their effect in guiding the fluid. Performance will inevitably fall off rapidly in a short time period, and the power consumed will also increase rapidly, so that the machine cannot sustain its performance.